The present invention relates generally to magnetic resonance imaging (MRI), and more particularly, to a method and apparatus for real-time localization, monitoring, triggering, and acquisition of MR images using a 3D real-time interactive imaging technique in MR studies utilizing both 2D and 3D images.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (GxGy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Three dimensional MR imaging techniques have been used routinely for high spatial resolution imaging (i.e., thinner contiguous slices, smaller voxel sizes, . . . ) due to an inherently higher SNR advantage over comparable 2D techniques. However, the scan time for 3D imaging is generally longer. It is therefore important to initially localize the volume of interest (VOI) accurately to avoid rescanning due to inaccurate or incomplete coverage of the VOI, especially when the VOI contains small anatomical structures such as found in the inner ears, breast lesions, liver metastases, etc. Additionally, three dimensional imaging of moving objects, such as the heart, coronary arteries, fetuses, etc. would be more efficient if real-time monitoring and localization were available prior to initiating a 3D acquisition. Furthermore, three dimensional breath-held and/or contrast enhanced imaging techniques require accurate timing and monitoring of the contrast agent to ensure consistent image quality.
Therefore, it would be advantageous to be able to localize and monitor in real-time the VOI prior to the triggering of the final, and typically much longer, three dimensional high resolution acquisition.